As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor is known. In the method, a radiation image storage panel containing a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at a considerably smaller dose, as compared with the conventional radiography. Therefore, the method is especially useful for direct radiography such as X-ray radiography for medical diagnosis.
The radiation image storage panel to be employed in the above-described method generally comprises a support and a stimulable phosphor layer provided on one surface of the support. However, if the phosphor layer is self-supporting, the support may be omitted.
As the stimulable phosphor layer, there are known not only a phosphor layer comprising a binder and a stimulable phosphor dispersed therein but also a phosphor layer composed of an agglomerate of a stimulable phosphor, containing no binder, which is formed by a deposition process or a firing process. In any of the above-described phosphor layers, the stimulable phosphor emits light (stimulated emission) when excited with stimulating rays such as visible light or infrared rays, after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in an amount proportional to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described hereinbefore, and it is desired for the radiation image storage panel to be employed in the method to have a high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.).
For enhancing the sensitivity of the radiation image storage panel, it is known that a light-reflecting layer is provided between the support and the stimulable phosphor layer by applying a coating dispersion comprising a binder and white pigment over the support. For example, a radiation image storage panel having a light-reflecting layer comprising a white pigment is disclosed in Japanese Patent Provisional Publication No. 56(1981)-12600. In the publication, titanium dioxide, white lead, zinc sulfide, aluminum oxide and magnesium oxide are described as examples of the white pigments.
As a stimulable phosphor to be preferably employed for a radiation image storage panel, a divalent europium activated alkaline earth metal (especially, barium) fluorohalide phosphor has been proposed because it gives stimulated emission of high luminance. The spectrum of the stimulated emission of this phosphor ranges from near UV region to blue region. In the spectrum, emission intensity in near UV region is stronger than that in visible region, and the peak of the spectrum is located around 390 nm.
In the case that a stimulable phosphor which emits stimulated emission in both UV region and visible region (e.g., above-mentioned divalent europium activated alkaline earth metal fluorohalide phosphor) is employed for a radiation image storage panel, the sensitivity of such panel can not be satisfactorily enhanced by the provision of a light-reflecting layer comprising white pigments exemplified in the Japanese Patent Provisional Publication No. 56(1981)-12600 except magnesium oxide between the support and the stimulable phosphor layer. This is because such white pigments have a considerably low reflectivity in near UV region while shows high reflectivity for the light in visible region. Therefore, the light-reflecting layer comprising such white pigments does not have sufficient reflectivity.
In order to improve a light-reflecting layer comprising white pigment, some studies concerning white pigment materials have been reported. One of such studies is disclosed in Japanese Patent Provisional Publication No. 59(1984)-162500, in which an alkaline earth metal fluorohalide represented by M.sup.II FX (wherein M.sup.II is at least one metal selected from the group consisting of Ga, Sr and Ca; X is at least one halogen selected from the group consisting of Cl and Br) is employed as white pigment.